Integrated circuit technology is very advanced in the area of discrete surface mount passive components (i.e., resistors, capacitors, and inductors). For example, this technology is very popular in mixed signal designs, such as, for portable wireless electronics and other devices in which digital and radio frequency (RF) circuits are combined into mixed signal modules. However, as the size of electronic devices decreases, it has become increasingly important for designers to optimize the available real estate on mixed signal chip modules. For instance, in some mixed signal designs, off-chip passive components use more real estate on the boards than the analog and digital signal processing units. By providing smaller passive components, designers may more efficiently use available real estate on boards or reduce the size of the boards themselves. Therefore, the development of relatively smaller passive components suitable for mounting to printed wiring boards has become increasingly important.
The use of existing systems and methods for implementing integrated passive components, however, may be problematic for several reasons. It is important to model the behavior of passive components, which are constituents in critical components such as filters, couplers, phase locked loops, etc., extremely accurately and in a reasonable processing time. The trade off of speed versus accuracy is one that has always plagued designers. Accordingly, it is a goal of the design community to develop solutions that are fast and accurate for modeling integrated components. Besides the difficulty in modeling integrated passives, the presence of severe parasitic effects in silicon-based RF IC's makes the design of high Q reactive components difficult. Q factor refers to the measure of “quality” of a particular frequency response. Therefore, it is advantageous to design integrated passive components having a high Q. Low temperature co-fired ceramic (LTCC) technology for multi-chip modules (MCMs) used in RF and wireless systems is one solution to this design problem of designing high Q integrated passive components. However, LTCC is an extremely expensive process to implement for consumer applications because of the complexity of the high-temperature fabrication process and/or the expense of the ceramic materials used in the substrates.
With specific regard to inductors, they form an integral part of filters, resonators, baluns, matching networks and bias networks. Inductors are commercially available as off-chip discrete components fabricated using multilayer ceramic substrates. Their construction generally has been limited to multilayer ceramic substrates because the ceramic materials used are resilient to moisture and temperature and show little variation with these parameters, which is imperative for inductors used in high frequency applications. There are essentially three types of ceramic inductors that are available in discrete form: winding internal construction, multilayer ceramic, thin film. The general properties of these inductors are provided below in Table 1.
TABLE 1Prior Art Ceramic InductorsStructureFeaturesSuitable applicationsSizesWindingHigh Q;IF Impedance(2.0 × 1.5 mm)InternalLarge current capacity (750 mA);matching;toConstructionlow tolerance (>25% variance),RF Oscillation(3.2 × 1.6 mm)low yield;circuit;Physically largeIF Choke;Circuits where largecurrents flow;Circuits where highQ characteristicsMultilayerLow current capacity (450 mA);RF/IF impedance(1.0 × 0.5 mm)CeramicInexpensive;matching;toGood high-frequency-rangeRF oscillation circuit;(1.6 × 1.8 mm)characteristicsRF chokeThin FilmPhysically small and thin; lowRF/IF Impedance(0.6 × 0.3 mm)current capacity (450 mA);matching;toLow L deviation;RF Oscillation(1.0 × 0.5 mm)Good high-frequency-rangecircuit;characteristics; high tolerance (2–RF Choke;5% variance); high yield;Circuits requiringExpensivetight inductancetolerance
None of the above-noted inductor designs provides a suitable combination of low cost, high yield, and high current. In addition, these designs do not provide in-built shielding, and therefore, if shielding is desired it must be added once mounted to the circuit board, typically in the form of a can or by adding references adjacent or below the device, which adds to the complexity of modeling the device (e.g., reduces quality factor). Alternatively, modeling the component and then modeling the circuit board with the component is very complex, as discussed in Wang et al., “A Full-Wave Analysis Model for Uniplanar Circuits With Lumped Elements,” IEEE Transactions on Microwave Theory and Techniques, Vol. 51, No. 1, pp. 207–215, January 2003.
Additionally, the high temperatures associated with the processing of ceramic-based inductors adds process complexity and thereby increases cost. Commercial manufactures currently utilize an automated system for a 5″×5″ wafer. Although the use of ceramic substrates for packages with embedded components and attached chips has been performed, it currently cannot be used for the replacement of an entire board for commercial wireless applications. LTCC modules are almost always mounted on a larger ceramic carrier, which is generally processed using organic thin-film laminate technology, which is currently automated for 18″×24″ wafers. This involves three levels of packaging, which is relatively expensive compared to other prior art inductors.
As an alternative to ceramic-based designs, organic materials have been utilized with varying degrees of success. The loss tangent for the dielectrics used in organic processes is typically anywhere from 0.02 for epoxy-based materials to as low as 0.0005 for teflon-based materials. Cost is crucial for widespread acceptance of a material, and epoxy-glass laminate is the lowest cost material and consequently holds a large market share in package substrates and printed wiring boards. However, these organic dielectrics exhibit frequency dependent elecrtrical behavior that is not suitable for broadband applications. Additionally, they have high moisture uptake and show high variance with temperature. On the other hand, materials such as teflon based composites, which exhibit resilience to temperature and moisture, are expensive and difficult to process. For example, currently the hardness of the teflon based composites, such as polytetra flouroethylene (PTFE), makes it difficult to fabricate small vias and through holes for selectively defining intra-layer connectivity because the PTFE melts from the high concentration of heat generated by the mechanical drill and then shatters. In addition, it is currently difficult to make multilayer structures out of PTFE because of its inertness. Those multilayer structures that comprise PTFE do not include layer-to-layer selective interconnectivity, but rather, are limited to through holes that extend from the top layer to the bottom layer.
Thus, there is a need in the industry for low-cost, high performance inductors for use in broadband applications.